Same Tires on 4x4 Vehicles Required?

[Joe2209]

Hi,

I have a 99 Ford Explorer XLT, and I recently purchased snow tires for the front. I didn't buy all four snow tires because snow tires are very expensive, especially if you're buying them for a truck.

The Explorer has automatic 4x4, which means if it senses I'm sliding, the 4x4 will automatically kick in. The tire place I purchased them from told me not to drive it, especially seeing as the 4x4 is automatic, because apparently it will damage my transmission. I made a few phone calls to a couple different places - some say it will damage the transmission and some say that it won't.

The tricky thing is, all four tires are the exact same size. The only difference is that the front tires (the snow ones) are a different brand, and of course, they're snow tires, while the back ones aren't.

I drove it all day today (and we just so happened to get a really bad snow storm too). The 4x4 has been kicking in and out all day, and I don't notice any problems - it shifts normally and everything. The only problem I've noticed (not sure if this is a problem, but I know nothing about vehicles) is that when the 4x4 is activated, I can hear a noise. I can't explain the noise - it's really weird - almost as if the emergency brake was applied when it's activated, only making the noise when I apply the gas or brakes.

Will driving my Ford Explorer ruin my transmission? Could these sounds I'm hearing relate, in any way, to the difference in the tires, or is this probably just a problem within the 4x4 itself?

If possible, I need quick answers. I'm going to need to know if I'll be able to take the truck to work tomorrow or not. If not, I'll have to call some people ahead of time to make arrangements.

Thanks for the help.

- Joe

 

[CapriRacer]

Joe,

First, there is no regulation governing the physical dimensions of a tire. So different tires may have different actual dimensions even though the tire size is the same. There is one manufacturer who produces 4 different sizes out of the same mold dimensions - meaning they split the difference in what the dimensions are supposed to be.

Second, the real issue is rolling diameter (or circumference), and even tires with the same unloaded diameter may have different rolling diameters differences. The differences would be small, but it brings me to the last point.

The vehicle's tolerance for differences in diameter is different from vehicle to vehicle. So you need to ask Ford, what their vehicle tolerates - and I'll bet they will give you the answer "only use 4 of the same size, type and manufacturer". which doesn't really answer your question at all.

So now it becomes a matter of risk. Are you willing to sacrifice a drivetrain for the cost of 2 tires? Want to know how big the risk is? Then you need to tell use what the unloaded diameters or circumferences are. I have heard that anything over 1/4" dimater might be a problem, but I just don't have enough vehicle experience to tell you - I'm almost strictly a tire guy.

 

[GregLocock]

Bear in mind that the rolling radius of your tire varies with loading, by far more than 1/8", yet somehow the trans copes with that

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Joe2209]

What about the noises I'm hearing? Is it possible these noises are caused by the difference in the tires, or could it possibly be something with the 4WD system?

 

[FoMoCoMoFo]

Hi Joe, have you measured the rolling circumference yet ?
If it's close tires are probably not a problem.
did the noise start when you changed tires or not until after driving all day on the new tires ?
as others have stated the system has to be able to tolerate some differences caused by normal tire wear,variations in loading, temp and air pressure and maybe the biggest going around a corner.
of course going around an occasional bend is not the same as many miles of high speed with different size tires.

 

[Joe2209]

The noises started when I began using the 4WD. The first day of using the system, the noises were very brief and weren't loud, but now it seems the noises are getting worse and louder.

I took it to work last night, and the Explorer was making the noise really loud while driving slowly on a clear road - I'm not sure if the auto 4WD was kicking in or not (not sure why it would seeing as the roads were clear).

It wasn't making the noises while driving 70mph on the highway - it only did it at low speeds (maybe 10-30mph).

Anyways, I purchased the same size, brand, and type of tire as the front ones. So now I have all four matching tires. It isn't making the noises anymore while driving slowly on a clear road, but I guess the true test is waiting until tomorrow when we get our snow storm so I can test out how the 4WD is working.

Thanks for all the help guys.

 

[Joe2209]

"The noises started when I began using the 4WD. The first day of using the system..."

I started using the 4WD system after I had purchased the front snow tires.

 

[cibachrome]

You could adjust the rolling diameter with differential air pressure. That would make the transfer case happy.

The main problem as I see it is that the delta rolling torques front to rear will strike out when you get onto a low-coefficient surface. One axle will give up.

You'll get a different truck out of the experience, though. Maybe a new one.

 

[Joe2209]

It's been running great the past few days in the snow. I also notice a difference in speedometer readings. Sometimes I'll monitor my speed by satellite (GPS System), and when I'm on the highway going 75mph, the speedometer reads 77-78mph. The readings are now 100% accurate!

Thanks again all for the help!